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11.10. Exercises 113

11.10 Exercises

Exercise 11.1. Write a function that reads the words in words.txt and stores them as keys in a
dictionary. It doesn’t matter what the values are. Then you can use the in operator as a fast way to
check whether a string is in the dictionary.
If you did Exercise 10.10, you can compare the speed of this implementation with the list in operator
and the bisection search.
Exercise 11.2. Read the documentation of the dictionary method setdefault and use it to write a
more concise version of invert_dict . Solution: http: // thinkpython2. com/ code/ invert_
dict. py .
Exercise 11.3. Memoize the Ackermann function from Exercise 6.2 and see if memoization
makes it possible to evaluate the function with bigger arguments. Hint: no. Solution: http:
// thinkpython2. com/ code/ ackermann_ memo. py .
Exercise 11.4. If you did Exercise 10.7, you already have a function named has_duplicates that
takes a list as a parameter and returns True if there is any object that appears more than once in the
list.
Use a dictionary to write a faster, simpler version of has_duplicates . Solution: http: //
thinkpython2. com/ code/ has_ duplicates. py .
Exercise 11.5. Two words are “rotate pairs” if you can rotate one of them and get the other (see
rotate_word in Exercise 8.5).
Write a program that reads a wordlist and finds all the rotate pairs. Solution: http: //
thinkpython2. com/ code/ rotate_ pairs. py .
Exercise 11.6. Here’s another Puzzler from Car Talk (http: // www. cartalk. com/ content/
puzzlers ):
This was sent in by a fellow named Dan O’Leary. He came upon a common one-syllable,
five-letter word recently that has the following unique property. When you remove the
first letter, the remaining letters form a homophone of the original word, that is a word
that sounds exactly the same. Replace the first letter, that is, put it back and remove
the second letter and the result is yet another homophone of the original word. And the
question is, what’s the word?
Now I’m going to give you an example that doesn’t work. Let’s look at the five-letter
word, ‘wrack.’ W-R-A-C-K, you know like to ‘wrack with pain.’ If I remove the first
letter, I am left with a four-letter word, ’R-A-C-K.’ As in, ‘Holy cow, did you see the
rack on that buck! It must have been a nine-pointer!’ It’s a perfect homophone. If you
put the ‘w’ back, and remove the ‘r,’ instead, you’re left with the word, ‘wack,’ which is
a real word, it’s just not a homophone of the other two words.
But there is, however, at least one word that Dan and we know of, which will yield two
homophones if you remove either of the first two letters to make two, new four-letter
words. The question is, what’s the word?
You can use the dictionary from Exercise 11.1 to check whether a string is in the word list.

To check whether two words are homophones, you can use the CMU Pronouncing Dictionary. You
can download it from http: // www. speech. cs. cmu. edu/ cgi-bin/ cmudict or from http:
// thinkpython2. com/ code/ c06d and you can also download http: // thinkpython2.
com/ code/ pronounce. py , which provides a function named read_dictionary that reads the
pronouncing dictionary and returns a Python dictionary that maps from each word to a string that
describes its primary pronunciation.